Carbon-coated Li-containing powders and process for production thereof

ABSTRACT

The invention provides a new route for the synthesis of carbon-coated powders having the olivine or NASICON structure, which form promising classes of active products for the manufacture of rechargeable lithium batteries. Carbon-coating of the powder particles is necessary to achieve good performances because of the rather poor electronic conductivity of said structures. For the preparation of coated LiFePO 4 , sources of Li, Fe and phosphate are dissolved in an aqueous solution together with a polycarboxylic acid and a polyhydric alcohol. Upon water evaporation, polyesterification occurs while a mixed precipitate is formed containing Li, Fe and phosphate. The resin-encapsulated mixture is then heat treated at 700° C. in a reducing atmosphere. This results in the production of a fine powder consisting of an olivine LiFePO 4  phase, coated with conductive carbon. When this powder is used as active material in a lithium insertion-type electrode, fast charge and discharge rates are obtained at room temperature and an excellent capacity retention is observed.

This application is a Divisional of U.S. application Ser. No.10/518,560, filed Aug. 31, 2005, the entire contents of which isincorporated herein by reference.

The present invention relates to the field of rechargeable lithiumbatteries and to positive electrode materials operating at voltagesgreater than 2.8 V vs. Li⁺/Li in non-aqueous electrochemical cells. Thisinvention relates in particular to the use of phosphates or sulphates oftransition metals as positive electrodes and allows the manufacturing ofpowdered Li-containing olivine-like and NASICON-like material, with theparticles efficiently coated with a controlled amount of conductivecarbon.

Lithium secondary batteries are now widely used in consumer electronics.They benefit from the light weight of Li and from its strong reducingcharacter, thus providing the highest power and energy density amongknown rechargeable battery systems. Lithium secondary batteries are ofvarious configurations depending on the nature of the electrodematerials and of the electrolyte used. The commercialised Li-ion system,for instance, uses LiCoO₂ and Carbon graphite as positive and negativeelectrodes, respectively with LiPF₆ in EC/DEC/PC as a liquidelectrolyte. The operating voltage of the battery is related to thedifference between thermodynamic free energies within the negative andpositive electrodes. Solid oxidants are therefore required at thepositive electrode, the materials of choice, up to now, being either thelayered LiMO₂ oxides (with M is Co or Ni) or the 3-dimensional spinelstructure of Li[Mn₂]O₄. Extraction of Li from each of these three oxidesgives access to M⁴⁺/M³⁺ redox couples located between 3.5 to 5 V vs.Li⁺/Li.

Three-dimensional framework structures using (XO₄)^(n−) polyanions havebeen proposed recently (U.S. Pat. No. 5,910,382) as viable alternativesto the LiM_(x)O_(y) oxides. LiFePO₄ and Li₃Fe₂(PO₄)₃ in particular arethe most promising Fe-containing materials that can work at attractivepotentials vs. Li⁺/Li (3.5 V and 2.8 V respectively). Both compoundsoperate on the Fe³⁺/Fe²⁺ redox couple which take advantage from theinductive effect of the XO₄ ^(n−) groups that diminishes the strength ofthe Fe—O bond compared to a simple oxide.

Pioneering work by Padhi (Padhi et al., J. Elec. Soc. 144(4))demonstrated the reversible extraction of Li from the olivine-structuredLiFePO₄ prepared by solid state reaction at 800° C. under Ar atmosphere,starting from Li₂CO₃ or LiOH.H₂O, Fe(CH₃COO)₂ and NH₄H₂PO₄.H₂O.Unfortunately, probably due to kinetic limitations of the displacementof the LiFePO₄/FePO₄ interface, only 60-70% of the theoretical capacityof 170 mAh/g of active material, was achieved, whatever the charge ordischarge rate applied. Indeed, the use of high synthesis temperaturesleads to the formation of large particles in which ionic and electronicconductivity is the limiting factor. Several research groups recentlyreported improvements in the effective reversible capacity of LiFePO₄ bydecreasing the particle size. This can be done by using highly reactiveFe^(II) precursors (JP 2000-294238 A2), or by using a solution route (WO02/27824 A1), thus allowing LiFePO₄ formation at lower temperaturescompared to the solid state route described by Padhi.

The poor electronic conductivity of the product can be improved bycoating the particles with conductive carbon. This has been done by ballmilling LiFePO₄ and carbon (Huang et al., Electrochem. Solid-StateLett., 4, A170 (2001)) or by adding a carbon containing compound toalready made LiFePO₄ and proceeding to a subsequent calcination at about700° C.(CA 2,270,771). Carbon, and preferably amorphous carbon, can alsobe introduced in the LiFePO₄ synthesis process, being mixed with thesolid synthesis precursors before calcination (EP 1184920 A2).

The main problems that may jeopardise the effective use in a positiveelectrode for Li batteries of Li-containing olivine or NASICON powderssuch as LiFePO₄ or other components mentioned by Goodenough et al. inU.S. Pat. No. 5,910,382, arises from their low electronic conductivityand from the fact that both end-members of the de-intercalation process(e.g. LiFePO₄ and FePO₄) are poor ionic conductors.

As described above, adding carbon, thereby coating the particles with aconductive layer, alleviates the electronic conductivity problem.However, high amounts of carbon are needed. Whereas carbon does notparticipate in the redox reactions useful for the operation of thebattery, a strong penalty for the overall specific capacity of thecomposite positive electrode is paid. This is illustrated in JP2000-294238 A2 wherein a LiFePO₄/Acetylene Black ratio of 70/25is used.

The ionic conduction problem can be solved by producing veryfine-grained particles. Using a solution route synthesis has been foundto be advantageous compared to the classic solid synthesis route. Thissolution route has been described in EP1261050. This route provides fora very finely divided, homogeneous precursor which needs only moderateconditions of temperature and time to react to the desired crystallinestructures. Thanks to the moderate conditions, grain growth, leading tounwanted coarse particles, is avoided. After synthesis, such a powderhas to be ball-milled with a relatively large quantity of conductivecarbon, typically amounting to 17 wt. %.

This invention provides for an improved solution route, ensuring theproduction of fine grained particles efficiently covered with aconductive carbon layer. Compared to prior art powders, the obtainedpowders deliver exceptional performances when used in Li-ion batteries.The invention provides for a powder that needs much less total carbon inthe electrode for a similar electrode capacity and discharge rate.Similarly, the invention provides for a powder that provides highercapacity and discharge rate when using the same amount of total carbonin the electrode.

A new process is presented for preparing a carbon-coated Li-containingolivine or NASICON powder, comprising the steps of

preparing a water-based solution comprising, as solutes, one or moreLi-containing olivine or NASICON precursor compounds and one or morecarbon-bearing monomer compounds,

precipitating a Li-containing olivine or NASICON precursor compounds andpolymerising the monomer compounds in a single step,

heat treating the obtained precipitate in a neutral or reducingenvironment so as to form a Li-containing olivine or NASICON crystallinephase and decompose the polymer carbon.

The process is specially suitable for the preparation ofLi_(u)M_(v)(XO₄)_(w) with u=1, 2 or 3, v=1 or 2, w=1 or 3, M isTi_(a)V_(b)Cr_(c)Mn_(d)Fe_(e)Co_(f)Ni_(g)Sc_(h)Nb_(i) witha+b+c+d+e+f+g+h+i=1 and X is P_(x-1)S_(x) with 0≦x≦1.

It is clear that the individual ‘a’ to ‘i’ parameters have values goingfrom 0 to 1. Obviously, their particular values should allow forelectroneutrality of the crystalline phase when combined with a properset ‘a’, ‘v’ and ‘w’ parameters. Examples are: LiMPO₄ such as inLiFePO₄, LiNiPO₄, LiMnPO₄; LiM₂(PO₄)₃ such as in LiTi₂(PO₄)₃,LiFeNb(PO₄)₃; Li₂M₂(PO₄)₃ such as in Li₂FeTi(PO₄)₃; Li₃M₂(PO₄)₃ such asin Li₃Ti₂(PO₄)₃, Li₃Sc₂(PO₄)₃, Li₃Cr₂(PO₄)₃, Li₃In₂(PO₄)₃, Li₃Fe₂(PO₄)₃,Li₃FeV(PO₄)₃.

The invented process is especially suitable for the preparation ofcoated LiFePO₄.

The precipitation of Li-containing olivine or NASICON precursorcompounds and the polymerisation of the monomers can be performed byevaporating water from the water-based solution. The carbon-bearingmonomer compounds can be a polyhydric alcohol and a polycarboxylic acid,such as, respectively, ethylene glycol and citric acid.

When the synthesis of coated LiFePO₄, is envisaged, equimolar amounts ofLi, Fe and phosphate, such as LiH₂PO₄ and Fe(NO₃)₃, are dissolved inwater together with a polyhydric alcohol and a polycarboxylic acid, thewater is then evaporated at a temperature between 60 and 100° C., and aheat-treatment is performed at a temperature between 600 and 800° C.,preferably between 650 and 750° C.

The object of the invention also concerns a carbon-coated LiFePO₄ powderfor use in Li insertion-type electrodes, which, when used as an activecomponent in a cathode cycled between 2.0 and 4.5 V against a Li anodeat a discharge rate of C/5 at 25° C., is characterised by a reversibleelectrode capacity expressed as a fraction of the theoretical capacityand a total carbon content of

at least 75% capacity and less than 4 wt. % carbon,

or,

at least 80% capacity and less than 8 wt. % carbon.

Other objects of the invention are: an electrode mix containing theabove-mentioned carbon-coated LiFePO₄ and batteries containing thelatter electrode mix.

For a proper understanding of the invention as described herein, thefollowing definitions are to be considered.

A “Li-containing olivine or NASICON precursor compound” is to beunderstood as a metal-bearing compound such as a salt, oxide orhydroxide of one or more metals susceptible to be converted to, or toreact to, the desired final compound. Typically, the conversion orreaction is performed by applying a thermal treatment

A “carbon-bearing monomer compound” is to be understood as an organiccompound susceptible to polymerise with itself (to form a homopolymer)or together with other monomers (to form a copolymer).

A “reducing environment” can be obtained by using a reducing gas, or byrelying on reducing properties of solids, such as carbon, present in thebulk of the material.

The “electrode capacity expressed as a fraction of the theoreticalcapacity” is the ratio of the capacity of the active product containedin the electrode, to the theoretical capacity of the active product. ForFeLiPO₄, a specific theoretical capacity of 170 mA/g is assumed.

When the charge or discharge rate is expressed as C/x, this means thatone Li per LiFePO₄ is exchanged in ‘x’ hour.

The general principle of the invention can be applied whenever a highquality carbon coating is needed on a metal-bearing powder. Olivine andNASICON phases, when used in rechargeable Li-ion batteries, are known tobe rather poor electronic conductors. As such, they particularly benefitfrom a carbon coating which is rendered conductive by a suitable heattreatment.

It is assumed that the metal bearing precursors, such as Li, metal andphosphate or sulphate ions, are trapped homogeneously on the atomicscale throughout the chelating polymer matrix. Such a structureeliminates the needs for long range diffusion during the subsequentformation of the crystalline phase. Therefore, at relatively lowtemperature, the precursors can form a homogeneous single phase ofprecise stoichiometry, intimately coated by a conductive carbonaceousnetwork.

Solvent evaporation conducting to an homogeneous mix of solid precursorcompounds and the polymerisation of the monomers are performed in onesingle step. This requires the polymerisation to occur simultaneouslywith the solidification of at least part of the precursor.

Different means can be employed to form the homogeneous mix of precursor(e.g. change in pH, temperature) and to trigger the polymerisation (e.g.addition of catalyst, UV). However, when the polymerisation reactionproduces water as a condensate, both the precipitation of the precursorand the polymerisation are triggered by identical means, i.e. by removalof water from the reaction vessel. This results in a particularly simpleand efficient process.

It has been found that the presence of heteroatoms (i.e. atoms otherthan C, O and H) in the monomers may degrade the performance of theobtained carbon coating, in particular its electrical conductivity. Itis therefore preferred to use monomer compounds containing only C, Q andH atoms.

When the production of LiFePO₄ is envisaged, the Fe source in theprecursor compound can be Fe^(II) or Fe^(III): the reducing conditionsneeded to avoid the burning of the carbon coating during the step ofheat treatment ensures the conversion of any Fe^(III) to the requiredFe^(II) state.

The preferred water evaporation temperature range is 60 to 100° C. Thisensures that the precipitation of the precursor compound and thepolymerisation reaction occur at least partly simultaneously.

The conductivity of the carbon residue is enhanced when the heattreatment is performed at 600° C. or higher. However, a temperature ofmore than 800° C. may degrade the quality of the product because ofgrain-growth or because of excessive reduction by carbon. A heathtreatment at 650 to 750° C. is preferred.

The positive electrode of the electrochemical cell is made of optimisedLiFePO₄ particles intimately mixed with an electronically conductingcarbon species made as described below. The activematerial/coated-carbon ratio can be adjust in the synthesis of LiFePO₄between 1 and 25 wt % of carbon. It is preferred to minimise therelative amount of carbon, whether present as coating material or ascarbon added during the manufacture of the electrode. Indeed, carbondoes not participate in the redox reactions and therefore representsinert mass reducing the specific capacity of the electrode.Nevertheless, it is desired to have at least 2 wt. % of coated carbon toexploit the invention fully.

The invention is illustrated by the preparation of optimised LiMPO₄particles, coated with (electronic) conductive carbon throughlow-temperature chemical routes.

For the preparation of a LiFePO₄/C composite, an aqueous solutioncontaining Fe, Li and phosphate is prepared using e.g. Fe(NO₃)₃.9H₂O andLiH₂PO₄. The solution is added under stirring in air to an aqueoussolution of citric acid. Ethylene glycol is then added to the solutionfor an ethylene glycol/citric acid molar ratio of 1/1. The precursor tocarbon ratio in the solution will determine the relative amount ofcarbon in the coating. Key to this process are the fact that both theLiFePO₄ precursors and the monomers are to be water-soluble.

In a second step, the water is slowly evaporated at 80° C. under air.When nearly dry, the solution turns to a gel due to the polymerisationbetween citric acid and ethylene glycol. The gel is dried by maintainingit at 80° C. A very homogeneous mixture, containing Li, Fe and phosphatein the stoichiometric proportions of LiFePO₄ together with the carbonbearing polymer, is then produced. Advantageously, monomers are chosenwhich have a lower partial pressure than water at the dryingtemperature. Premature evaporation of the monomers is thus avoided.

In a third step, the homogeneous mixture is progressively heat-treatedunder a reducing atmosphere (N₂/H₂, 10% H₂) to yield, at a temperatureof about 500° C., a crystalline LiFePO₄ phase coated with a controlledamounts coated carbon. However, at 500° C., the coated carbon is partlyinsulating. A treatment between 600° C. and 800° C. is thus preferred asit yields conductive carbon. Thanks to the presence of carbon, thesurrounding environment of LiFePO₄ is strongly reducing. This is usefulto reduce remaining traces of Fe^(III) precursors to Fe^(II), but canlead to unwanted results when the percentage of carbon is high. Indeed,high carbon contents (more than 15%) combined with prolonged treatment(more than 5 hours) at 700 to 800° C. partly reduces Fe^(II) in LiFePO₄to Fe⁰. This leads to the formation of impurities such as Fe₂P. Asdetermined by electrochemical titration, the obtained optimised powdermay still contain a small amount of Fe^(III) (less than 3 M %), anamount which is in fact inferior to that obtained in the synthesis ofpure LiFePO₄ without carbon. The result of the heat treatment can easilybe monitored and optimised by e.g. X-ray diffraction or by Mossbauerspectroscopy, to ensure that Fe^(III) is nearly completely reduced toFe^(II) and that no significant amount of Fe^(II) is reduced to Fe⁰.

The invention is illustrated by the following examples. Four LiFePO₄/Ccomposites were produced according to the process described above.Aqueous solutions containing 0.4 M/1 Fe, Li and phosphate and 0.1 to 1M/1 ethylene glycol and citric acid were prepared using Fe(NO₃)₃.9H₂Oand LiH₂PO₄. The solutions were dried for 12 h at 80° C. The dryresidues were then heat treated for 10 h at 700° C. under a N₂/H₂atmosphere with 10% H₂.

The results, presented in Table 1, show the influence of the monomerconcentrations in the solution on the amount of carbon coated on theLiFePO₄ particles. The apparent loss of carbon, which is rather highcompared to the theoretical amount expected, comes probably from thereduction of Fe^(III) to Fe^(II) during the heat treatment. Thepolymerisation needs not be complete.

TABLE 1 Theoretical vs. observed amount of carbon in the coating as afunction of the monomer concentration in the solute (for 0.1 M/l of Fe,Li and phosphate in the solute) Citric acid Ethylene glycol TheoreticalC Observed C (M/l) (M/l) (wt. %) (wt. %) 0.1 0.1 13.2 0.33 0.2 0.2 23.33.6 0.4 0.4 37.8 8.6 1 1 60.3 24

FIGS. 1 to 5 illustrate the invention.

FIG. 1: X-ray diffractograms (CuKα) and the S.E.M. photographs of twoLiFePO₄ powders coated with 3.6 (top) and 24% (bottom) carbon

FIG. 2: Electrochemical response of a Li/LiPF₆ EC:DMC/LiFePO₄electrochemical cell (swagelok type) cycled at C/5 and 25° C., usingLiFePO₄ with 3.6 (top) and 24% (bottom) of coated carbon

FIG. 3: Results obtained with Li/LiPF₆ EC: DMC/LiFePO₄ electrochemicalcoin cells embedded in a plastic film. LiFePO₄ with 3.6% of coatedcarbon cycled at C/5 and 25° C. (A) or 55° C. (B); LiFePO₄ preparedaccording to the prior art solution route and ball-milled with 17% ofconductive carbon cycled at C/10 and 55° C. (C)

FIG. 4: In situ ED patterns of LiFePO₄ in a Li/LiPF₆ EC: DMC/LiFePO₄electrochemical cell cycled at C/5 and 25° C.; LiFePO₄ preparedaccording to the invention (top) and according to the prior art solutionroute and ball-milled with 17% of conductive carbon (bottom)

FIG. 5: Evolution of the specific active material capacity achieved in aLi/LiPF₆ EC:DMC/LiPePO₄ prepared according to the invention with 3.6 (B)and 24% (C) of coated carbon; LiFePO₄ prepared according to the priorthe art solution route and ball-milled with 17% of conductive carbon(I); commercial LiCoO₂ are shown for comparison (A)

FIGS. 1 to 5 are now discussed in more details. The X-ray diffractogramsand the S.E.M. photographs of two LiFePO₄ powders coated with 3.6 and24%.of carbon are given in FIG. 1. The photographs are representativefor the overall powder. For LiFePO₄ with 3.6% of coated carbon, thenetwork formed by the coated particles is very well spaced and regular.The particles are sufficiently fine (around 1 μm) to alleviate thepenalising displacement length of the interface between LiFePO₄ andFePO₄, while enough space is left for species to migrate. For 24% ofcoated carbon, the carbon matrix itself can be observed. The carbonnetwork surrounds the LiFePO₄ particles whose size is even smaller thanin the former case. The LiFePO₄ phase appears to be pure when 3.6% ofcarbon is coated. When 24% is coated, some LiFePO₄ is reduced to Fe₂Pafter 10 h at 700° C. This demonstrates that the higher the carbonpercentage, the more efficient the reduction.

These powders give the electrochemical response shown in FIG. 2. Theelectrochemical cells were built in Swagelok configuration with Li metalpasted on a Ni foil as the negative electrode, and LiPF₆ in EC:DMC asthe electrolyte. The positive electrode is the powder obtained directlyfrom the described process. The signature of FIG. 2 (voltage as afunction of x in Li_(x)FePO₄) was obtained at 25° C. for an equivalentcharge/discharge rate of C/5, i.e. I Li extracted or inserted in 5 h.

About 85% of the theoretical capacity of the active material can beachieved when using 24% of coated carbon. The performance of the totalelectrode is however rather penalised by the large quantity of carbon.The amount of carbon can be dramatically decreased. When using 3.6% ofcoated carbon, 78% of the capacity is still achieved. In each case, theirreversible capacity at first cycle is very small.

FIG. 3 illustrates the stability of the LiFePO₄ composite producedaccording to the invention using 3.6% of coated carbon. This materialwas cycled at C/5 at 25 and at 55° C. The resulting specific capacity issuperior to that obtained with uncoated material prepared according tothe prior art solution route and ball-milled with 17% of conductivecarbon. If we compare the specific capacities of the total electrodes,the superiority of the invented process becomes even more apparentthanks to the much lower amount of total carbon.

In FIG. 4, in situ X-ray diffraction patterns are shown for a fullcharge/discharge cycle. With the powder coated according to theinvention, at the end of the charge cycle, all the diffraction peaks ofLiFePO₄ disappear at the benefit of triphylite-FePO₄ peaks. The biphasicphenomenon is thus complete. However, with powder prepared according tothe prior art solution route, this is not the case.

In FIG. 5, the specific capacity of several active materials is reportedin function of the cycling rate. Materials tested are: LiFePO₄ compositeobtained by the process according to the invention with 3.6 and 24% ofcoated carbon, LiFePO₄ prepared according to the prior art solutionroute and ball-milled with 17% of conductive carbon, and commercialLiCoO₂. The 3.6% carbon-coated LiFePO₄ performs better than any other atlow discharge rates. At higher rates, it is outperformed by LiCoO₂ (amuch more expensive product), and, as expected, by 24% carbon-coatedLiFePO₄. Indeed, the higher amount of coated carbon tends to improve thehigh current performance. Whatever the conditions, however, the productswhich are carbon-coated according to the invention remain superior tothe prior art product.

1. A carbon-coated LiFePO₄ powder for use in Li insertion-typeelectrodes, which, when used as an active component in a cathode cycledbetween 2.0 and 4.5 V against a Li anode at a discharge rate of C/5 at25° C., is characterized by a reversible electrode capacity expressed asa fraction of the theoretical capacity and a total carbon content of atleast 75% capacity and less than 4 wt. % carbon, or, at least 80%capacity and less than 8 wt. % carbon.
 2. Electrode mix containingcarbon-coated LiFePO₄ for use in Li insertion-type electrodes, which,when used as an active component in a cathode cycled between 2.0 and 4.5V against a Li anode at a discharge rate of C/5 at 25° C., ischaracterized by a reversible electrode capacity expressed as a fractionof the theoretical capacity and a total carbon content of at least 75%capacity and less than 4 wt. % carbon, or, at least 80% capacity andless than 8 wt. % carbon.
 3. A battery containing an electrode mixcontaining carbon-coated LiFePO₄ for use in Li insertion-typeelectrodes, which, when used as an active component in a cathode cycledbetween 2.0 and 4.5 V against a Li anode at a discharge rate of C/5 at25° C., is characterized by a reversible electrode capacity expressed asa fraction of the theoretical capacity and a total carbon content of atleast 75% capacity and less than 4 wt. % carbon, or, at least 80%capacity and less than 8 wt. % carbon.